The present invention relates to a length- or angular measuring instrument. Measuring instruments of this kind are primarily used to determine the relative displacement or absolute position of movable machine parts, for example, in machine tools or measuring machines.
In length and angular measuring instruments of the foregoing type, a material measure, e.g., in the form of an optical grating, is scanned by a sensor which is mounted behind a scanning plate constituted as another grating. An exact adjustment of the angle between the two gratings is necessary, in this context, to avoid the moirxc3xa9 effect produced when working with gratings which are tilted towards one another, in that the two gratings are positioned in parallel, or to intentionally produce a moirxc3xa9 effect in that a specific angle is precisely adjusted. When a plurality of marks is applied to the material measure, the effect of angular errors which are too great is that the relationship among the marks is lost. In some situations, a reference mark is no longer able to be allocated to the correct period of an incremental graduation.
To adjust the moirxc3xa9 angle between the material measure of a scale and the scanning plate of a scanning unit, German Published Patent Application No. 28 44 066 describes a lever arm which is secured, on one side via a flector joint to a movable or stationary machine part and, on the other side, has an adjustment screw, which is supported on this machine part and which bears the scanning head, together with the scanning plate. Thus, when the screw is adjusted, the lever arm, together with the scanning unit, swivel, so that the moirxc3xa9 angle is adjusted, in turn.
A disadvantage of this system is that it does not permit a precise adjustment of the moirxc3xa9 angle. Given a typical lever arm length of 50 mm and an adjustment screw having a screw pitch of 0.8 mm, an angular change of 16 mrad results for one turn of the screw. Moreover, in this system, the lever arm is only protected in one direction against vibrations. For that reason, the scanning unit must be secured by an additional clamping screw.
It is conventional, as a fine-adjustment gear, to use a dual screw gear having a differential screw, as described, for instance, by S. Hildebrand: Feinmechanische Bauelemente (xe2x80x9cPrecision-Mechanical Componentsxe2x80x9d), Hanser Publishers, Munich, Vienna 1978, 3rd edition, pp. 534-536. A dual screw gear permits a precise adjustment in the linear direction, since, in the context of one screw turn, the change in the distance between two objects joined by the differential screw corresponds to exactly the difference between the two pitches of the differential screw.
It is an object of the present invention to provide a length or angular measuring instrument which may permit a fine adjustment of the moirxc3xa9 angle, a low-vibration attachment of the scanning unit, and thereby may make do with few components.
This objective is achieved by providing a device as described herein. Further example embodiments of the present invention are also described herein.
A lever, which is secured via a flector joint to the fixed or movable machine part, is able to be adjusted in that, instead of a conventional screw resting on this machine part, a differential screw is provided, which is screw-coupled by its first thread to the lever and, by its second thread, to this machine part. If, at this point, one turns the screw by one rotation, the distance of this lever end to this machine part changes precisely by the difference in the pitches, when the directional sense of the pitch is the same. As a result, the lever changes its inclination, since it is executing a rotary motion about the flector joint. In the above mentioned example, given thread pitches of 0.7 mm and 0.8 mm, a change in distance of 0.1 mm results for one rotation of the differential screw, and thus an angular change of approximately 2 mrad. Thus, the adjustment may be carried out in a more precise manner than conventionally possible. Since the dual screw coupling may provide protection from vibrations in two directions, the need may be eliminated for an additional clamping of the lever. In addition, the differential screw is pretensionsed or biased by the axial forces which arise in response to deformation of or strain on the flector joint. Moreover, the differential screw is clamped by the mutual tilting of its two drilled holes. It may thereby be screw-coupled in a self-locking manner.
Further aspects of, as well as details pertaining to the present invention are set forth in the following description of the Figure, which shows a design in accordance with the present invention of a device for adjusting a scanning head.